Topic 3: Thermal physics
3.1 Thermal concepts
3.1.1
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3.1.2
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3.1.3
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3.1.4
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7 hours
2 hours
State that temperature determines the direction of thermal energy transfer between two objects.
Temperature is a measure of the average kinetic energy of the vibrations called thermal motion.
When moving particles collide, energy is transferred from the one with more KE to the one with less KE.
Thermal energy is transferred from the object at the higher temperature to the object at the lower temperature until the two are in
thermal equilibrium when at the same temperature.
State the relation between the Kelvin and Celsius scales of temperature.
A temperature scale is constructed by choosing two reproducible events such as the melting of ice and the boiling of water. The
temperature values of these two events are then chosen.
The Celcius scale is based on the temperature of melting ice being assigned the value zero. This is an arbitrary zero.
The Kelvin temperature scale starts at absolute zero and each degree is the same as the Celcius scale.
Kelvin = Celsius + 273
State that the internal energy of a substance is the total potential energy and random kinetic energy of the molecules
of the substance.
The thermal motion of particles is the total of the kinetic energies of movement and rotation.
Potential energy is stored in the forces between particles.
The internal energy of the substance is the total of the potential and kinetic energies of the particles of the substance.
Explain and distinguish between the macroscopic concepts of temperature, internal energy and thermal energy (heat).
Thermal energy refers to the non-mechanical transfer of energy between a system and its surroundings.
o It is incorrect to refer to the “thermal energy in a body” as it would be to refer to the “work in a body”.
Temperature is a number that determines if internal energy will be transferred to or from the object and its surroundings.
Heat is infra-red electromagnetic waves that travel at the speed of light.
o When heat is absorbed, the internal energy of a substance increases.
o When heat is radiated from a substance, its internal energy decreases.
Thernal Energy is transferred by three modes:
1. Conduction occurs when a temperature difference exists between two substances or within the one substance.
o Collisions cause the higher kinetic enregy particles to slow down and the lower kinetic energy particles to speed up.
o Solids are the best thermal conductors: particles are stacked close together and collisions occur readily between them.
2. Convection currents form when a heat source causes the speed of particles in a liquid or gas to increase.
o The faster moving particles migrate upward and the slower particles move in underneath to take their place.
o This causes a current of faster particles moving up and a current of slower particles moving down.
3. Radiation is the transfer of energy by infra-red radiation.
o Infrared radiation is released by particles of one object causing them to slow down
o This infrared radiation is absorbed by the particles of another object causing them to speed up.
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3.1.5
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3.1.6
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Define the mole and molar mass.
A mole of a substance is the mass that has the same number of particles as are in 12 g of the C-12 isotope.
A mole of a gas has a mass in grams equal to the sum of the atomic weights of the atoms in one molecule.
Define the Avogadro constant.
A mole of a gas contains 6.023 x 1023 particles. This is Avogadro's number.
3.2 Thermal properties of matter
5 hours
Specific heat capacity, phase changes and latent heat
3.2.1
Define specific heat capacity and thermal capacity.
3.2.2
Solve problems involving specific heat capacities and thermal capacities.
3.2.3
Explain the physical differences between the solid, liquid and gaseous phases in terms of molecular
structure and particle motion.
3.2.4
Describe and explain the process of phase changes in terms of molecular behaviour.
Students should be familiar with the terms melting, freezing, evaporating, boiling and condensing, and should be able
to describe each in terms of the changes in molecular potential and random kinetic energies of molecules.
3.2.5
Explain in terms of molecular behaviour why temperature does not change during a phase change.
3.2.6
Distinguish between evaporation and boiling.
3.2.7
Define specific latent heat.
3.2.8
Solve problems involving specific latent heats.
Problems may include specific heat calculations.
Kinetic model of an ideal gas
3.2.9
Define pressure.
3.2.10 State the assumptions of the kinetic model of an ideal gas.
3.2.11 State that temperature is a measure of the average random kinetic energy of the molecules of an ideal gas.
3.2.12 Explain the macroscopic behaviour of an ideal gas in terms of a molecular model.
Only qualitative explanations are required. Students should, for example, be able to explain how a change in volume
results in a change in the frequency of particle collisions with the container and how this relates to a change in
pressure and/or temperature